1. Field of the Invention
The invention relates to a device for the temperature surveillance of a print head or of a hammer block constructed with an electromagnetic coil and which is used in particular for serial matrix pin printers.
2. Description of the Background of the Invention Including Prior Art
A print head or a hammer block constructed with an electromagnetic coil represents an interface between the mechanics of a matrix printer and the electronics of the print-head control. The demands for higher and higher print speeds and device lifetimes result in an extreme mechanical and electrical load onto the print head. These requirements result in that the print head becomes one of the most expensive device components in a printer. In particular, in the case of impact printers, substantial electrical control powers are necessary for the printing, which is usually performed by passing current through a coil, and these electrical control powers are eventually transformed into heat and heat dissipation occurs. These power losses increase in proportion to the number of print elements and the control speed of the print elements. Consequently, it is necessary to monitor the temperature of the print head in order to avoid a thermal destruction of print-head device components. Upon reaching of a temperature critical for the print head, usually the print speed is reduced by a lowering of the operating frequency or the print head is temporarily switched off until the print-head temperature has cooled to a defined value, or a blower motor is turned on for providing cooling.
A temperature-sensitive sensor is used for the surveillance. The sensor transforms the print-head temperature into a corresponding measurement value. Based on space and cost reasons, one single sensor is employed and disposed centrally relative to the print head. The sensor determines the heat losses of all individual magnet coils resulting in a mixed average temperature value. The determined average temperature value depends in such cases on the temperature drop between the electromagnetic coils and the sensor, the time elapsed for the heat transport, the number of the electromagnetic coils controlled. Consequently the measurement value determination can be quite inaccurate relative to the purposes for which it is performed.
It has been proposed in the German Patent Application Laid Open DE-OS 2,618,224 to increase the number of print elements in the print head by at least one additional print element in a structure for increasing the lifetime of a print head for printing and writing apparatus, in particular for matrix printers, such that the printing power and capacity available is fully used only at rare times. However, in this case there occur problems relative to the frequency distribution of the feeding in of the current into the electromagnetic coil, associated with the print element, which cannot be determined in advance.
Another conventional structure taught in U.S. Pat. No. 4,496,824 controls a blower motor via a digital bus system, where initially the temperature, measured via a central thermistor, is determined via a voltage difference and an analog digital converter, and wherein the voltage difference is digitalized. This measurement is also relatively inaccurate based on the individual measurement result because the thermistor of the control circuit is mounted on the print-head surface. There is thus present a non-impact print head which itself generates heat during printing. Consequently, it is supposed that this structure does not carry an electromagnetic coil.
In the presence of a digitalized temperature measurement value, the switching-on and the switching-off of the print head of a printer can be performed according to a further teaching via special programs as referred to in the European Patent Application 0,176,732.
A particular problem is represented by a printer operating in graphic mode, where there can occur a particularly non-uniform and/or one-sided use load of the print head, for example because only one single print element is continuously operated In this case, there occur local overheating situations of the electromagnetic coil, whereby any kind of conventional temperature surveillance fails to provide adequate protection for the print head from thermal destruction.